1. Field of the Invention
This invention relates to an apparatus for optically automatically examining the surface of a substrate, and more particularly to an apparatus for automatically detecting a foreign particle such as minute dust adhering to a substrate on which a semiconductive integrated circuit pattern is provided, such as a photomask for LSI, a reticle or a wafer.
2. Related Background Art
As an apparatus of this type, there is known an apparatus in which, as disclosed in U.S. Pat. No. 4,468,120, a laser beam applied to a reticle or the like and the difference between the directional characteristics of scattered lights from a circuit pattern and a foreign particle is utilized to automatically detect only the foreign particle. The principle of this known apparatus is as follows.
When a light beam condensed and stopped down, for example, a laser beam, is applied to the edge portion of a circuit pattern (a thin film layer of chromium or like material) on a substrate, there is created a scattered light of strong directionality having had its directionality determined by the angle of incidence onto the edge. On the other hand, when a foreign particle is present on the surface of the substrate and a laser beam is applied thereto, the scattered light spreads in all directions.
The scattered light thus created is detected by a condensing lens and a photoelectric detector. If a plurality of photoelectric detectors are disposed so as to look into that portion of the substrate to which the laser beam has been applied, from different directions, and their photoelectric outputs are compared, the scattered light from the circuit pattern edge and the scattered light from the foreign particle can be discriminated therebetween.
However, in the prior art as described above, there is no problem when the circuit pattern on the substrate is sufficiently larger than the laser beam spot and when the circuit pattern is of the same size as the laser beam spot, whereas the scattered light from the circuit pattern loses its directionality when the circuit pattern is sufficiently smaller than the laser beam spot, and this has led to the problem that it becomes difficult to discriminate between the circuit pattern and the foreign particle.